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This report compares the predictions of the Health Protection Agency (HPA) application of the ‘R91’ model with those of the UK Met Office NAME III model. The study considers a simplified application of NAME and R91 to enable a fair model comparison.

The comparison is centred upon analysis of model output generated from a single baseline run for a short duration release of the type often considered in emergency response assessments. Subsequent model runs are performed, scoping a range of model scenarios and commonly modified model input parameters. Differences in the predictions of the two models are investigated and explained. The quantitative assessment of differences in the baseline model output is used as part of a qualitative assessment of observed differences across a range of model runs and their associated output.

There is a disparity (of up to a factor of approximately 3) between time-integrated activity concentrations in air derived using NAME and those derived using R91, most notably in the near-field. R91 is more conservative in its approach, and estimates made by R91 are typically greater than those made by NAME. The primary sources of the observed differences between the model outputs are identified as:

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